Tidal currents offer a considerable source of sustainable energy at various sites throughout the world, usually within easy reach of land and in relatively shallow waters. Tidal currents are created by movement of the tides around the earth producing a varying sea level, dependent on the phases of the moon and sun. As the sea levels vary, so the waters attempt to maintain equilibrium subject to gravitational forces, thus inducing flow from one area of sea to another. This flow is modified by a number of factors such as, the Coriolis forces due to the earth rotation, earth/moon/sun alignment, local topography, atmospheric pressure and temperature and salinity gradients. The major advantage of tidal power generation is its regularity, which can be predicted for years in advance.
According to a study by the ETSU (Energy Technology Support Unit) the United Kingdom may obtain up to 20 percent of its total electricity by using these systems to collect energy from fast moving tidal currents that exist in channels and offshore areas. Similar resources have been noted to exist elsewhere such as in the Straits of Messina, between Sicily and mainland Italy.
The most powerful flows tend to occur in areas of restriction, either by width or depth, but for the same reasons are not suitable for widespread exploitation by large, fixed devices which require a minimum rotor area, and therefore water depth, to justify the costs of installation and maintenance. It is assumed from the outset that new tidal barrage systems are unlikely ever to be pursued due to their inherent properties of high cost, delayed financial return, and serious environmental consequences.
The considerable size of the available resource has attracted various proposals for its exploitation.
The following represents the existing systems within the field of tidal current energy extraction. It is assumed that power transmission problems will be equal for any system, and that all systems will require some form of non-toxic anti-fouling agent.
There also exist operational environmental impacts common to all methods of tidal power generation, such as, an inherent risk of collision damage to fish and marine mammals, redirection of currents and the sediments and food particles contained within them, and shipping, particularly fishing.
A first type of tidal current energy extraction system encountered on the market is the Monopile system. This technology is well known and understood by contractors familiar with the offshore oil industry. It consists of twin axial flow turbines, each turbine driving a generator via a gearbox, mounted on streamlined cantilevers either side of a circular section, vertical steel monopile. It is anticipated that a number of structures will be grouped together in ‘farms’. The planning of such a tidal ‘farm’ would need to be accurately modelled for wake effects, as once installed, the monopile is expensive to re-site. In addition, operational depth is restricted to the 20 m-35 m range. Concerning the installation and maintenance, monopile systems require a hole to be drilled in suitable bedrock and the base of the turbine tower is secured within the socket so produced. Existing monopile support mechanisms for presenting a tidal turbine to the tidal currents are expensive, thus making only a few sites economically viable for power generation and requiring considerable sub sea engineering expertise.
The current monopile systems permit raising the turbines above water level for maintenance and repair, which is beneficial, but the long-term (i.e. 20 years) reliability and corrosion resistance of the necessary mechanism must be questionable. The protrusion of the piles above sea level would reduce the likelihood of impact with passing vessels.
Concerning the environmental and decommissioning issues, the impact of installation would be considerable, especially to the benthic flora and fauna, but subsequently the piles may become areas of shelter and therefore, populated. To minimise the danger to shipping and fishing, decommissioning would require complete removal of the piles, which would disturb the benthic population once again.
A second type of tidal current energy extraction system that exists in the prior art is the floating tether. This floating tether device is anchored to the seabed with a mooring cable and suspended clear of the seabed using a flotation buoy. The axial flow tidal current turbine is free to position itself into the direction of the tidal flow, which obviates the need for a yaw mechanism.
Several prototypes have already been developed including a 10-kilowatt device tested in Scotland in 1994. At present, the arrangement is unlikely to be suitable for large power output installations due to the relative sizes of anchor, turbine and float. On occasions of relatively high velocity tidal streams (e.g. spring tides), if the anchor holds, the turbine will be dragged lower in the water with the unwanted potential to collide with the seabed.
Concerning the installation of the floating tether system, it is relatively quick and inexpensive. However, visual inspection would need to be frequent as the structure is likely to be subject to storm damage and fatigue loading of the cable, leading to possible loss of the supporting float and subsequent sinking of the device, or loss of anchorage and subsequent drifting. Once sunk, the device would be open to damage by the oscillating tidal currents and could prove difficult to recover, whilst a drifting device would potentially cause damage to any other moored turbines in its path.
Due to the length of tether required and the random positioning of the device at any one time, this arrangement is not suitable for closely grouped tidal farms and a safe spread would fail to make economical use of the power available in a given area. For the same reasons, this type of arrangement would present a hazard to all forms of shipping, large and small. It would, however present a possible solution to a one-off, small scale installation in areas such as the mouth of a sea loch. Concerning the environmental impacts of installation and decommissioning of the floating tether systems, it will be minimal, leaving no footprint on removal.
A third type of tidal current energy extraction system that also exists in the prior art is the oscillating hydroplane system. In that system, a central post mounted on five legs supports a complex mechanism comprising two interconnected symmetrical hydrofoils. These hydrofoils are used to pump high-pressure oil, which drives an electrical generator via a hydraulic motor. At the end of each stroke, the hydrofoils are tilted to give the required angle of attack to produce the return stroke, thus creating an oscillating motion.
Concerning the installation and maintenance, at present, the oscillating hydroplane system does not yet possess a launch and recovery mechanism. As a result of the constant oscillations and considerable number of moving parts, it is probable that this device will be subject to high dynamic loading and subsequent fatigue stress. The upward stroke of the hydrofoils will tend to lift the device off the seabed and hence increase the possibility of it being washed away at high tidal stream velocities.
Concerning the environmental impacts of installation and decommissioning of the oscillating hydroplane systems, they are expected to be minimal, leaving no footprint on removal. However, this cannot be confirmed until a launch/recovery mechanism is proposed. Using high pressure oil as a means of power transmission does however introduce the possibility of pollution in the event of leakage.
Some ‘tidal’ energy extraction systems can also be used in freshwater applications such as rivers.
With these existing systems and designs, it is a problem that their instabilities during operations as well as during launch and recovery, if possible, might cause damage. In addition, since these systems are becoming larger and larger, the frequent installation and maintenance operations will become more and more difficult and expensive.